(a) Fields of the Invention
The present invention relates to nitride semiconductor devices and their fabrication methods. In particular, the present invention relates to nitride semiconductor devices applicable to field effect transistors or semiconductor light emitting devices and their fabrication methods.
(b) Description of Related Art
Gallium nitride (GaN)-based semiconductors (nitride semiconductors) have wider band gaps than compound semiconductors such as gallium arsenide (GaAs) or semiconductors such as silicon (Si). Thus, on the GaN-based semiconductors, active research and development is being carried out toward realization of light emitting devices capable of emitting blue or green visible light or ultraviolet light. Hitherto, the GaN-based semiconductors have been put into commercial production as light emitting diode devices for various types of displays or semiconductor laser devices for next-generation high-density optical disks.
The GaN-based semiconductors are characterized by high breakdown voltage and high saturated drift velocity, so that they also receive attention as materials for electronic devices such as high-output power devices or high-speed transistors. The GaN-based semiconductor is generally formed on a so-called hetero-substrate made of a different material from the nitride semiconductor, such as sapphire (single crystal Al2O3) or silicon carbide (SiC). Initially, it was difficult to improve the crystallinity of the nitride semiconductor grown on the hetero-substrate. However, the technique in which a low-temperature buffer layer is used in a metal organic chemical vapor deposition (MOCVD) method was developed, and this technique has made it possible to provide a relatively good nitride semiconductor crystal by using the hetero-substrate.
In a heteroepitaxial growth technique that makes it possible to put an optical device such as a blue light emitting diode device into commercial production, any nitride semiconductor grown has the principal surface with an orientation of (0001) plane, that is, c-plane, and thus, in the nitride semiconductor, spontaneous polarization or piezoelectric polarization occurs in a perpendicular direction to the principal surface. Such polarization generates, in a quantum well structure generally used as an active layer in the light emitting device, a polarization electric field in a well layer with electrons and holes confined therein. This results in spatial separation of electrons and holes, so that a so-called quantum-confined Stark effect, which causes a decrease in light emission efficiency, is observed (see, for example, S. F. Chichibu et al., “SPECTROSCOPIC STUDIES IN InGaN QUANTUM WELLS”, MRS Internet J. Nitride Semicond. Res. 4S1, G2.7 (1999)).
Also, in an electronic device, a nitride semiconductor layer is generally formed on the (0001) plane. Thus, the spontaneous polarization or piezoelectric polarization mentioned above generates a polarization electric field perpendicularly to the (0001) plane. Moreover, this polarization also generates fixed charges at the interface and surface of the nitride semiconductor layer. For neutralization of these charges, for example, at the heterointerface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN), a sheet carrier concentration of, for example, 1×1013 cm−2 or higher is produced although the interface is undoped.
Hitherto, it has been reported that a heterojunction field effect transistor with a large drain current produced by utilizing this high sheet carrier concentration is realizable (see, for example, M. Hikita et al., IEDM Tech. Digest 2004 p. 803). In this document, it has also been reported that electrons therein have a mobility as high as above 1000 cm2/Vs at room temperature, so that great expectation grows on it as a future electronic device as an alternative to the conventional GaAs-compound semiconductor device or Si semiconductor device.
However, as described above, since the conventional GaN-based light emitting device and GaN-based field effect transistor are both formed on the (0001) plane of the nitride semiconductor layer, spontaneous polarization and piezoelectric polarization occur perpendicularly to the (0001) plane. Therefore, for the light emitting device, the quantum-confined Stark effect mentioned above makes it difficult to improve the efficiency of light emission from a quantum well, and thus there are disadvantageous limitations in further intensity and efficiency enhancements.
For the field effect transistor, if it is desired to obtain the normally-off characteristics that is strongly demanded of power devices, resulting polarization electric field described above produces the sheet carrier concentration to the transistor. This disadvantageously makes it difficult to attain the normally-off characteristics.